IP Multimedia (IPMM) is an example of a service that provides a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media that it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, e.g. peer-to-peer multimedia communication, IPTV etc.
These services can be based on the IP Multimedia Subsystem (IMS) architecture, which is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks (3GPP TS 22.228, TS 23.228, TS 24.229, TS 29.228, TS 29.229, TS 29.328 and TS 29.329 Releases 5 to 7).
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a 3GPP PS access domain.
When a user wishes to access any network from a fixed line, the user may first be authenticated in an access network using a Network Attachment Sub-System (NASS) (ETSI TS 282 004 v1.1.1). The NASS allocates an IP address to the fixed line in use, and authenticates and authorises the user. The NASS can also be used to configure preferences in the access network, depending on a user profile stored by the NASS.
When authenticating the end user in the access network, the user's terminal sends an in-use Line Identifier (LID) associated with the fixed line that the terminal is using to access the network. The LID is forwarded to a NASS entity called a Connectivity Session Location and Repository Functions (CLF). The CLF then associates the user's LID with the user's assigned IP address.
For users accessing an IMS network from a fixed line, ETSI TS 183 033 TISPAN defines a method of authentication for IMS users. This method is known as NASS-IMS bundled (NAB). The NAB method allows the IMS layer to re-use existing Network Attachment Sub-System (NASS) authentication status, as illustrated in FIG. 2. The IMS network performs a Line ID check, and if it is successful then the TISPAN IMS network trusts the access network authentication status and marks the user as authenticated.
In more detail, upon IMS-SIP registration, a Proxy-Call Session Control Function (P-CSCF) in the IMS network queries the CLF in the access network to retrieve the in-use LID and the IP address assigned to the in-use line. The P-CSCF uses the retrieved IP address in the SIP Register as a query key. The in-use LID is inserted into a SIP Register message using the P-Access-Network-Info (PANI) parameter of the SIP Register message. The REGISTER message containing the LID value is then sent to a Serving-Call Session Control Function (S-CSCF). When the S-CSCF performs a Multimedia Authentication Request (MAR) operation, it may not know the authentication scheme in use, and so the S-CSCF sets the Authentication-scheme Attribute Value Pair (AVP) in the MAR to “unknown”. The MAR is sent to a Home Subscriber Server (HSS), which selects an authentication scheme based on the subscribed authentication method. The user's IMS Private Identity (IMPI) and IMS Public Identity (IMPU), that are used in the SIP Registration procedure, are included in the MAR sent towards the HSS.
The HSS retrieves a registered Line Identifier (LID′) from a database. LID′ is a registered line identifier that is associated with the user and stored in the user's profile.
When the HSS responds to the S-CSCF with a Multimedia Authentication answer, the HSS sets the Authentication-scheme AVP to “NASS Bundled”, and includes LID′ in the Multimedia Authentication answer.
The S-CSCF compares LID′ returned by the HSS with LID received from the accessing terminal. If LID matches LID′, then S-CSCF sends a SAR message to the HSS. In this case, the user is considered successfully authenticated. The signalling sequence described above is illustrated in FIG. 3.
A problem with this solution is that it relies upon a correct association between the in-use Line Identifier (LID) value obtained from the CLF in the NASS, and registered Line Identifier LID′ stored in the user's IMS profile.
Referring to the example scenario illustrated in FIG. 4, a nomadic user makes use of a third party's fixed-line broadband connection (for example, the user may be a guest the third party's home and make use of the available DSL line or Fiber line). IMS registration relies on the registering user having an IMS identity, which is typically stored on a card. In order to register with an IMS network, the user must send their identity to the network. The nomadic user attaches to a fixed line connection in any one of a number of standard ways. For example, the nomadic could plug a 3G-enabled laptop into a modem, make use of an available WLAN router, or insert their smart-card/SIM-card/ISIM-card into local equipment. If authentication in the conventional NASS via the User Access Authorisation Function/Profile Database Function (UAAF/PDBF) is successful, the user can use the fixed-line broadband connection regardless of the user's IMS credentials. However, if the nomadic user then decides to register with an IMS network to make use of IMS services, the user sends its IMS Private Identity (IMPI) and IMS Public Identity (IMPU) to the IMS S-CSCF, according to 3GPP TS 24.229 V7.4.0 (2006-06). The S-CSCF sends the following information to the UPSF/HSS as part of the registration:                Authentication-scheme AVP set to “unknown”        User's IMPI and IMPU        
If the user attempting IMS registration has a NASS-Bundled authentication scheme enabled in his subscription, then the HSS/UPSF will return the user's registered LID′ back to the S-CSCF. However, the in-use LID differs from LID′ stored in the user profile, as the user is not using a fixed-line connection registered to that user. As a consequence, the S-CSCF cannot authenticate the user and notifies the user terminal accordingly. Further attempts by the terminal to register with the IMS network may be interpreted by the IMS network as a Denial of Service attack or fraud attempt, resulting in the IMS end user being blacklisted or blocked. This situation is not solved by ETSI TS 183 033 TISPAN.
This problem can occur whenever an IMS user (for which NAB authentication is enabled) makes use of a fixed line connection with an in-use LID value that is different from the registered LID′ value associated with the user in their IMS profile and stored in the HSS, for example, whenever an IMS user makes use of a visited WLAN/Wimax access point (which ultimately connects to a network using a fixed line) as a guest user.